1. Field of the Invention
The present invention relates generally to processing between two fluid streams of different compositions, and particularly to a membrane module for separation, purification, mass transport, exchange, or other types of processing applications of process streams involving the use of a purging, sweep or any other type of a second fluid stream.
2. Technical Background
Cross-flow filtration is known where the permeate flow conduits are disposed perpendicular or transverse to the feed flow conduits. Purge flow to sweep the permeate flow conduit is also known for disk-shaped, single tube-shaped, or bundled tubular membranes, such as hollow fibers. However, known single or bundled ceramic hollow-fibers are too fragile to be of practical use. Hence, cross-flow filtration devices with a purge access are known but they are typically based on polymeric hollow-fibers. Known polymer-based hollow fiber membrane modules have the desired high separation surface area per unit volume and permeability for effective mass transfer or exchange between two kinds of fluid streams suitable for separation, filtration, and extraction processes. One fluid stream flows inside the hollow fiber, while another fluid stream flows around the hollow fiber.
A number of individual small hollow fiber tubes have been grouped together to form a bundle. The void space among the individual fibers in a bundle provides space large enough for the fluid to move around and provide purge flow access. A purge flow formed on the bundled hollow fibers has also been known to improve membrane performance of the bundled hollow-fibers arranged in a cross-flow pattern.
However, the main problems with polymer materials are their susceptibility to attack by organic solvents and their instability at high temperatures (e.g. >250° C.) and other harsh operating environments. The porous polymeric support is not suitable for supporting inorganic membrane materials.
Many adsorbent materials known in the field, such as zeolites, provide the selective adsorption function. Zeolite membrane has been an active technology area for recent ten years, because it has a large potential for a variety of application. The salient feature of zeolite membranes is a well-defined pore structure that makes it possible to achieve real molecular sieving effect. For example, diffusion rate of linear-shaped molecules in the zeolites crystal could be several orders of magnitude higher than that of the corresponding isomer in a branch-shaped form. However, inorganic zeolites can not be applied to the incompatible organic polymeric material.
In theory, the hollow fiber tubes individual, their membrane, or both can be made of ceramic oxides, instead of a polymeric material, to offer the same surface area advantage and eliminate the problems associated with the polymeric bundled fibers. Besides, the ceramic membrane may offer high selectivity and flux on the basis of unit separation area. Further, the ceramic membrane may be regenerated.
However, the ceramic hollow fibers could not be made with the required mechanical strength and flexibility in an economical way. The hollow-fiber ceramic tubes have a high surface area but are fairly brittle. The inorganic membranes have poor mechanical flexibility, low membrane surface area per unit module volume, difficulty in membrane processing and module assembly, and high capital costs.
It is known that by embedding cross-flow channels in a strong porous body, the cross-flow module offers a separation area comparable to the hollow-fiber but with greatly enhanced mechanical strength and can be made cost effective. However, the key problem is how to manage the flow into or out of the embedded membrane channels or tubes, when there is no open or free space available between the embedded tubes. As is known, the bundle of hollow fibers or a single channel offers a much larger open space among individual hollow fibers for the flow to go around than the multi-channels being embedded in a porous but solid matrix.
For the membrane module of simple structure, such as planar disks or single-channel tubes, the adsorption and purge operation can be readily performed in the space available.
The disk or single-channel membrane module is commonly used in the laboratory and in some industrial processes. However, the disk-shaped or single-channel tubular inorganic membrane module has a low separation surface area per unit volume. One way to increase the surface area is to have many smaller feed channels in one inorganic membrane body. Such a monolithic design presents a challenge in how to introduce the sweep flow stream for an efficient removal of the adsorbed or permeated species. In a multi-channel, monolithic membrane module, the permeate is driven from the inner channels to the outside of the module by a pressure differential. When a purge flow is introduced to the outside of the membrane module, the purge flow cannot get into the inner channels because of the opposite pressure resistance. As a result, only the outside of the module surface is “purged”, while the permeate from the inner channels could not be reached.
Hence, as a general concept, using sweep flow for membrane separation is known. Using a monolith support is also known but the combination of an external sweep flow purge access in a monolith is not known. The challenge is how to conduct the purge of a membrane module with a number of flow channels being embedded in a solid but porous matrix. For a monolithic membrane module, the conventional purge method allows only for the sweep of the exterior-surface of the monolithic module only and the innermost membrane channels inside the matrix cannot be purged. How to purge every membrane channels in a monolith membrane support is the challenge.
Hence, there is a need to provide purge flow access in a low-cost and strong inorganic cross-flow filtration device having a high contacting area between two fluids, a high permeability through the membrane divider, long-term stability at high temperatures, ruggedness for operations under harsh environments, inertness toward the solvent attack, and resistance toward high pressure drops.